I don't know about the 500SEC, but when my 95 E420 heat was not working, I was getting a DTC 31 on socket 16 of the 38-pin connector. That indicated a bad mono valve. After replacing the valve, I had heat again. It only took 45 minutes to change the valve as it sits right behind the module blower box.
Do you have a code reader to scan for trouble codes?
Here's a good link to replacing it. http://www.*********.com/forums/showthread.php?4535-HOW-TO-Replace-Monovalve-1995-E420&highlight=valve
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1993 400E, 256,000 miles (totaled)
1994 E420, 200,000+ miles
1995 E420, 201,000 miles
Last edited by emerydc8; 10-17-2013 at 08:47 PM.
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